What equipment is shed at the fourth level of integrity?

The correct choice regarding the equipment shed at the fourth level of integrity is the cabin bus systems. In the context of aircraft systems and operational integrity levels, each level represents a tier of systems that may be shed to maintain operational control and safety during an in-flight emergency.

At the fourth level, the cabin bus systems are typically considered non-essential for flight safety and can be powered down to conserve energy or manage load when other essential systems are still operational. The focus at this integrity level is on maintaining critical systems that ensure safe operations, such as flight controls and navigation, while relinquishing power from less critical systems that do not directly affect the aircraft's ability to fly safely.

Maintaining the integrity of the primary and essential systems during scenarios that require load shedding is crucial for ensuring that the pilot can continue operations without compromising safety or functionality.